As an integrated circuit becomes highly integrated and highly dense, it is required for a projection aligner (an exposure apparatus) employed in semiconductor manufacturing to project and expose a circuit pattern of a reticle surface onto a wafer surface at higher resolution. Since a projection resolution of a circuit pattern depends upon the numerical aperture (NA) of a projection optical system and an exposure wavelength (λ), a method wherein the NA of a projection optical system is increased while having a fixed exposure wavelength, has been applied to manufacturing products. As an example of employing 248 nm, that is, a KrF excimer laser oscillation wavelength, an aligner having an NA of 0.6 or more has already been introduced commercially.
As the NA increases, an optical depth decreases, becoming inversely proportional to NA2. To solve this problem, a CMP process, that is, a planarizing technique, is introduced.
FIG. 1 shows the processing steps of metal CMP as an example of the CMP process, and a structure of a planarized wafer. According to the metal CMP shown in FIG. 1, etching is performed on SiO2 of the surface of a wafer (first step); then Tungsten is deposited on the surface of the wafer (second step); unnecessary Tungsten is removed by CMP processing (third step); Al/Si/Cu is deposited thereupon (fourth step); and TiN is deposited (fifth step).
In a planarization process of CMP, particularly, in the metal CMP shown in FIG. 1, it is difficult to maintain the most appropriate condition due to an apparatus factor error. Therefore, it is necessary to control parameters of a CMP apparatus so as to keep a uniform shape of a pattern of the wafer by frequently measuring the shape of the pattern after CMP.
For instance, such error causes deterioration of the alignment accuracy in a semiconductor aligner. More specifically, a structure of alignment marks becomes asymmetrical due to the CMP processing. As a result, a rotating error shown in FIG. 2 or a magnification error shown in FIG. 3 is generated in global alignment, causing deteriorated accuracy.
FIG. 4 shows data obtained by actually measuring alignment marks with an AFM. The data is obtained from resist-coated alignment marks. The alignment mark has a structure called metal CMP shown in FIG. 1. The right and left shots (patterns A and C) and the middle shot (pattern B) of wafer W are compared with respect to the resist shapes of the alignment marks. As can be seen in FIG. 4, the shape on the surface of the middle shot is symmetrical, but each shape on the surfaces of the right and left shots is asymmetrical. The patterns A and C have a reversed symmetry. If global alignment is performed with the reversed asymmetry as mentioned above, a rotating error shown in FIG. 2 is caused.
For this reason, in order to maintain high accuracy of alignment, it is necessary to control the CMP process by measuring the alignment marks after the CMP process with a profiler, such as an AFM, or the like, to make sure that the resist shapes are symmetrical. Besides the aforementioned symmetry of the pattern, it is also necessary to control characteristics such as planarity, over/under etching, dicing, and oxide erosion.
The stereoscopic shape measuring means (profiler) employed in the process control of the CMP can be categorized into optical means and non-optical means. For non-optical means, an AFM and a stylus profiler, and for optical means, a profiler employing an interferometer, are described below by citing specific examples.
An example of an AFM is a Dimension Metrology AFM, manufactured by Digital Instrument, which is aimed to measure critical dimensions (CD), e.g., a line width, pitch and depth, wall angle, roughness, and so on. A structure of this apparatus is shown in FIG. 5. Detection in the vertical direction of this drawing is realized by having the end (radius 5-20 nm) of a probe 9 approach to the surface of a measurement target 1 up to a point that an atomic force is generated, and detecting this position of the probe 9 with an optical sensor 10 (vertical resolution of 0.8 nm). Detection in the horizontal direction is realized by driving the probe (horizontal resolution of 1 nm) in the range of up to 70 μm with a piezoelectric device. Accordingly, stereoscopic measurement of the measurement target 1 is realized.
An effective measurement mode is neither a contact nor a non-contact mode, but a tapping mode, which realizes measurement by oscillating the probe 9 with a resonant frequency of 200 to 400 Khz.
An example of a stylus profiler is an HRP240ETCH (HRP: High Resolution Profiler), manufactured by KLA-Tencor, which is capable of measuring the entire area of a 300 mm wafer. A structure of this apparatus is shown in FIG. 6. Detection in the vertical direction of this drawing is realized by tracing the surface of the measurement target 1 with the end (radius 20-40 nm) of a stylus 13, which has the same function as that of the probe in the AFM, with a very low stylus force, and detecting the position of the stylus 13 with an electrostatic capacitance sensor 14 (vertical resolution 0.02 nm). Detection in the horizontal direction is realized by employing a combination of two stages: a stage (not shown) driven (horizontal resolution 1 nm) by a piezoelectric device for measuring a micro area of up to 90 μm and a motor-driven stage (not shown) for measuring a macro area of up to 300 mm. Accordingly, stereoscopic shape measuring is realized.
Furthermore, there is a mode called a dipping mode, where the stylus 13 is brought down to a sample set at a measurement point and brought into contact until a predetermined stylus force is achieved, then after measurement, the stylus is elevated to be moved in the measuring direction and again brought down. The dipping mode enables sample measurement with a high aspect ratio.
An optical non-contact type detection profiler includes types adopting various optical methods. One of them is New View manufactured by Zygo, which is a three-dimensional surface structure analyzing microscope. Detection of this microscope is realized by employing a Mirou interferometer as shown in FIGS. 7 and 8. As shown in the drawings, an interferometer-type objective lens 7 is constructed with a half mirror 11 and an internal reference mirror 12 for producing a reference beam. The reference beam and reflection light from the surface of the measurement target 1 are extracted by a beam splitter 5, and an interference figure is generated on a photoreceptor (CCD camera 8) positioned on a surface of the measurement target 1. In this apparatus, a halogen lamp serving as a white light source is used as the light source 4 to pick up a white interference figure. However, with an addition of a wavelength filter, monochromatic illumination may be used to enable topological measurement. For vertical detection, the objective lens 7 is driven by a piezoelectric device of a vertical scan driving unit 6, and the driving position is controlled in a closed loop by an electrostatic capacitance sensor (not shown), and in addition, interference figures on plural focus surfaces are inputted to a computer to be subjected to frequency domain analysis (FDA) using fast Fourier transformation (FFT), which is uniquely developed by Zygo, to obtain height data at a vertical resolution of 0.1 nm. A horizontal resolution (maximum of 0.1 μm/pixel) and a detection range in the horizontal direction are determined by a pixel pitch of the CCD camera 8 and an image forming magnification between the measurement target 1 and CCD camera 8 in the optical system.
In the process control of CMP, a profiler employed may be of an optical type or a non-optical type, such as an AFM or a stylus profiler, or the like, as long as it satisfies specific accuracy. To give priority to a resolution in the horizontal direction, non-optical profilers, such as an AFM or a stylus profiler, are advantageous.
The reason thereof is in that since a resolution in the horizontal direction is determined by an optical condition (NA, λ) of an optical profiler, resolution of 100 nm or less cannot be achieved. On the contrary, for instance, an end of a probe of an AFM, which possibly comes in contact with a sample (measurement target), has a radius of 5 to 20 nm, thus, apparently has a superior horizontal resolution to that of an optical profiler.
For the above-described reason, it is a current situation to use a non-optical profiler, such as an AFM or a stylus profiler, to achieve measurement of the horizontal direction at the highest resolution and to control processing of CMP, or the like, with high accuracy.
In addition, as an integrated circuit becomes highly integrated and highly dense as mentioned above, accurate alignment of a reticle and wafer is required. In order to meet this demand, Japanese Patent Application Laid-Open (KOKAI) No. 2000-228356 discloses a method of using a surface shape measuring system, such as an AFM, or the like, to calculate an alignment offset value after a surface-film is formed on a mark-formed portion of a measurement target, and to detect the position of the film-formed mark by using this offset value.
However, if an AFM or a stylus profiler is employed to give priority to a resolution in the horizontal direction, there is a possibility of contamination of a wafer surface.
When using an AFM or a stylus profiler, there is a case that a probe used for measurement strongly comes in contact with the wafer surface when it is not in use.
For instance, an AFM uses an atomic force as a measurement value, as its name implies. When the probe of the AFM moves from the current measurement point to a next measurement point, the distance between the probe and wafer becomes close due to a concave and a convex shape of the alignment mark on a wafer as shown in FIG. 9. Since the probe strongly comes in contact with the wafer surface, a predetermined atomic force is not generated. Therefore, the probe is elevated to a distance in which an atomic force is generated, and a measurement value is obtained. Although the circumstance depends upon the shape of an alignment mark, there are many cases that the distance varies more than a distance where an atomic force is generated. Therefore, in reality, such a circumstance of the probe strongly contacting the measuring surface often happens. If this causes a change in the shape of the probe, the measurement value changes. This is a factor that determines the life of a probe. The more micro the end shape of the probe is, the shorter the life of the probe is.
Herein, a probe is made of silicon. If this probe strongly comes in contact with a wafer used in CMP process control, the possibility of contamination cannot be denied. Furthermore, in a case of measuring a resist surface, a strong contact of a probe may damage the resist surface, as a resist is softer than silicon.
For this reason, in the current CMP process control, the wafer measured by an AFM, or the like, is discarded after measurement, without being returned to the process line, since it may be contaminated. Although the contamination may completely be eliminated by cleaning the wafer after measurement, the time necessary for cleaning causes a reduced throughput in the entire semiconductor device production line.